Why is disease control so important?

For wheat and barley crops to realise their full potential, both in terms of yield and quality, it is very important that a comprehensive fungicide programme is used to control the main diseases likely to infect your crops.

By physically damaging leaves and the ear, disease infection results in less green leaf area and therefore reduced yield and often reduced quality of yield as well. 

Diseases infection results in damage to leaves in different ways. Most diseases lead to unrepairable physical loss of leaf tissue. Powdery mildew, where the majority of the fungus exists on the leaf surface, is really the only cereal disease in New Zealand that doesn’t lead to green leaf loss once controlled. 

In addition to leaf loss resulting from the actual disease lesion, diseases often affect an area around the physical lesion causing it turn yellow, or lesions coalesce together resulting in large areas of dead leaf. This is often described as the “necrotic halo” and can be seen quite clearly with leaf rust infections.

Why does loss of green leaf area reduce yield?

Plants produce yield by healthy leaves capturing the energy from sunlight to produce a range of sugars which are stored as grain – the crop yield. However, not all leaves capture equal amounts of energy and it is important to understand the impact both the crop and crop growth stage have on sunlight interception and hence yield production.


For a wheat crop ninety percent (90%) of the final yield is produced by the top three leaves with the flag leaf and ear being responsible for over forty percent (40%) of yield.

To understand why this is it can be helpful to consider a wheat plant as a cylinder, an upright tube, with all of the upper leaves about the same size. Then it becomes apparent that with crop canopy shading reducing sunlight transmission down to the lower leaves the upper leaves, especially final leaf two and the flag leaf, along with the ear, are the yield power house of a wheat crop.

Wheat Yield Arable Fungicides Bayer

To illustrate this, studies were carried out in the United Kingdom and these established that a healthy wheat crop will develop 0.2 t/ha/day of yield. For each tonne of yield you are targeting your crop needs to stay green for five days from flowering.


Due to the lower leaves being larger, and greater light penetration due to the canopy structure resulting from the barley plant’s triangular shape, a higher proportion of a barley crop’s yield is produced by the lower leaves. This yield, which is produced early in the season, is initially stored in the stem, being transported to the ear later in the season as the crop matures.

Barley Yield Arable Fungicides Bayer

What are fungicides and how do I use them effectively?

Fungicides are products used to control disease infecting crops in an environmentally acceptable manner. Bayer has a powerful portfolio of fungicides – Aviator Xpro, Delaro, Folicur, Proline and Prosaro -that together very effectively control all of the key cereal diseases you will encounter in New Zealand.

Cereals grown in New Zealand are attacked by a range of globally important diseases. Diseases such as septoria leaf blotch, stripe and leaf rust and tan spot of wheat and scald, net blotch and Ramularia leaf spot of barley.

While all of these diseases can lead to significant yield losses the impact on yield can vary between different diseases which means some diseases are more important to control than others. Septoria leaf blotch and scald are two particularly devastating diseases which if left uncontrolled will lead to yield losses of up to 40% or more.